Typical multi-plate clutches operate by the pressing of a piston inside, a casing or drum, against a number of separator plates and friction plates to engage the clutch. The pressing of the friction plates between the separator plates causes the separator plates to engage a hub, which consequently transmits turning force from an input to an output.
In many transmissions the piston slowly spins or rotates within the drum when pressure is applied by the piston to the separator plates and friction plates and when the transmission as a whole vibrates. This spin or rotation causes the friction plates to undergo rotation and cause the friction material on them to wear down. This spin or rotation also causes the separator plates to overheat, causing “blue” discolorations in the material.
A typical piston 2 is shown in FIG. 1. As can be seen from this figure, piston 2 includes a first surface 4 configured to face the interior surface of a drum (not shown) and a second surface 6 configured to face one or more clutch plates. Another view of piston 2 is shown in FIG. 2, with second surface 6 visible. The outer circumferential surface of surface 6 is smooth and is capable or rotating or spinning within a drum during operation.
What is desired is a piston that will overcome these premature wearing issues in multi-plate clutches.